1. Field of the Invention
The present invention relates to a telescopic antenna for a so-called "hand-held type" portable telephone which is installed in the portable telephone for receiving call signals and transmitting and receiving ultrashort waves for the purpose of communications with correspondents.
2. Prior Art
FIGS. 7(a) and (b) respectively show conventional examples of telescopic antennas for portable telephones.
The telescopic antenna for a portable telephone shown in FIG. 7(a) is constructed in the following manner: an external antenna 2 consisting of a whip antenna is attached to the upper wall of the portable telephone housing 1 in such a manner that the antenna 2 is freely inserted into and extended from the housing 1. An internal antenna 3 for receiving incoming calls which responds to call signals is mounted inside the housing 1, and these external and internal antennas 2 and 3 are connected via a change-over switch 4 that performs a change-over operation linked to the telescopic operation of the external antenna 2, so that selective change-over operation is performed with respect to a transmitter-receiver 5 installed inside the housing 1.
In the telescopic antenna for the portable telephone shown in FIG. 7(b), an external antenna 2 consisting of a whip antenna is, as in the case of FIG. 7(a), attached to the upper side wall of the portable telephone housing 1 so that the antenna 2 can be freely inserted into and extended from the housing 1. An inverted F-form antenna 6, which doubles as an impedance matcher and an internal antenna, is provided inside the housing 1. The F-form antenna 6 is electrostatically coupled to the external antenna 2 via a capacitance 7. When the external antenna 2 is extended (or extended from the housing), the inverted F-form antenna 6 is caused to act as an impedance matcher. When the external antenna 2 is retracted (or pushed into the housing), the inverted F-form antenna 6 is caused to act as an internal antenna for incoming calls.
In both of these conventional antennas shown in FIGS. 7(a) and (b), the internal antenna used for incoming calls is installed inside the portable telephone housing 1. As a result, the housing 1 needs to have a relatively large internal volume. Such an increase in the size of the housing 1 runs counter to the compactness and reduced weight, which are strongly demanded as essential features of portable telephones, and should therefore be avoided as much as possible.
Furthermore, as to the current distribution on the conventional antennas when the external antenna 2 is extended from the housings, it is found that the current distribution is based on the top of the housing 1, regardless of the electrical length of the antenna (3.lambda./8, 5.lambda./8, 3.lambda./4, etc.) that corresponds to the wavelength of the telephone waves used. In other words, electromagnetic waves are radiated directly from the top of the housing 1.
In the conventional antenna-installed portable telephones constructed as described above, some of the electromagnetic waves emitted by the antenna radiate into the human body (especially into the head, which is close to the antenna) when the portable telephone is held in the hand and the receiver is placed against the ear. As a result, some of the radiant energy is absorbed and attenuated by the heat, etc., so that there is a danger that the sensitivity of the portable telephone will drop during use. Recently, electromagnetic waves at gigahertz frequencies have been used as telephone waves. Accordingly, it is feared that the radiation of such high electromagnetic waves may have a deleterious effect on the human body.